(a) Field of the Invention
The present invention relates to an objective lens system for an endoscope and, more particularly, to an objective lens system for an endoscope with an extremely high magnification (for example, from 30.times. to 40.times.).
(b) Description of the Prior Art
By endoscopes, observation by magnifying the object is being performed in recent years and, moreover, there is increasing demand for observation at a very high magnification at a level similar to observation by microscopes. Among known objective lens systems for endoscopes of magnifications 10.times. to 30.times. for ordinary observation, there are so-called variable magnification lens systems which are arranged that the lenses constituting the lens system are partially shifted along the optical axis of the objective so that it is possible to observe the object by further enlarging it, as the objective lens system approaches the object, by making the magnification higher than those of ordinary objective lens systems. However, in case of those lens systems, the magnification of the objective lens system itself is less than 2.times. even when the objective lens system is put to the position where the distance to the object is the shortest and, in practice, the magnification of the optical system as a whole is made high by multiplying the magnification of the eyepiece so that the magnification of the endoscope as a whole becomes 10.times. to 30.times. as mentioned in the above.
Among known endoscopes for microscopic observation, there is no endoscope which is designed for the purpose of observation at a high magnification only, and there are only such endoscopes which are arranged that observation at a high magnification is made possible by the way of use.
The above-mentioned way of use is to use an endoscope composed as shown in FIG. 1 of the present application like the endoscope disclosed in Japanese published examined patent application No. 31555/73. That is, in a distal end 1 of an endoscope, an objective lens system 2 and cover glass 3 are arranged as shown in FIG. 1, an optical fiber bundle 4 for observation is arranged in rear of the objective lens system 2, an optical fiber bundle 5 for illumination is arranged in parallel with the optical fiber bumdle 4 for observation, and the endoscope optical system is composed of them. Observation by the above-mentioned endoscope optical system is performed by putting the outer surface 3a of the cover glass 3 into direct contact with a mucous membrane surface 6. Besides, fixing needles 7 are provided to the front surface 1a of the distal end 1 in order to keep the distal end stable when the front end (cover glass 3) of the endoscope is put into close contact with the mucous membrane surface 6. As another way of use, Japanese published examined utility model applications No. 49823/76 and No. 20168/82 disclose such endoscopes arranged that a miniature microscope, which comprises an objective lens system, an optical fiber bundle for observation, and an eyepiece, is inserted into an inserting passage of an endoscope, which comprises an observation optical system, an illumination optical system, and an inserting passage, in order to observe an object at a high magnification.
The documents of known endoscopes cited in the above do not disclose any concrete example of an objective lens system having high magnification. Besides, up to the present, there exists no known objective lens system for an endoscope having a high magnification.